To a small saucepan over medium heat, add the apple cider. Cook for 20-22 minutes, or until it is reduced to about ¼ cup. Set aside.
Preheat oven to 350°F (175°C). Line a 9x9-inch baking dish with parchment paper. Set aside.
In a large bowl, whisk together the melted butter and light brown sugar until fully incorporated.
Add the eggs and the egg yolk, one at a time, whisking well after each addition.
Whisk in the vanilla extract.
Stir in the reduced apple cider until well combined.
In a separate medium-sized bowl, whisk together the flour, cornstarch, baking powder, and salt.
Stir the flour mixture into the butter and egg mixture until completely combined.
Pour the batter into the prepared baking dish in an even layer.
In a small bowl, combine the sugar and apple pie spice. Whisk to combine. Sprinkle evenly over the blondie batter.
Bake for 35-38 minutes, or until the edges just begin to turn golden brown and the center is no longer jiggly.
Allow the bars to cool completely in the pan on a wire rack.